1. Field of the Invention
The present invention relates to a drum brake apparatus having an automatic clearance adjusting device that adjusts position of brake shoes in accordance with a wear, and more particularly to an improvement for making it easier to assemble the apparatus into a vehicle and simplifying a clearance adjustment between a drum and each brake shoes after assembling them to the vehicle.
2. Description of the Related Art
Different kinds of conventional drum brake apparatuses are used for braking a vehicle in the travel. The drum brake apparatuses are for example classified into leading trailing type drum brake, two-leading type drum brake, and duo-servo type drum brake that are depended on the position of the brake shoes pressed against the inner circumferential surface of a drum in an approximately cylindrical shape.
When a drum brake apparatus of any of the types described above is provided in a vehicle, an automatic clearance adjusting device is typically incorporated. The adjusting device automatically adjusts the position of the pair of brake shoes that are provided and opposed in the internal space in the drum in accordance with the wear of the brake shoes so that the clearance between the brake shoes and the drum in a non-braked state is not increased by the wear.
Conventional automatic clearance adjusting devices can roughly be classified into micro-adjuster type that can only slightly adjust the feed amount of the brake shoes by a single occasion of braking operation (see for example Japanese Patent Examined Publication JP-B-56-34733 and JP-B-63-33567) and one-shot adjuster type that adjust the clearance between the brake shoes and the drum to a reference value at a time by a single occasion of braking operation (see for example Japanese Patent Examined Publication JP-B-43-21822, JP-B-51-12782 and JP-B-2-21456).
In a drum brake apparatus having the automatic clearance adjusting device of the conventional micro-adjuster type, however, when the initial clearance between the drum and brake shoes is set large for the ease of assembling them to a vehicle, braking operation must be repeated many times in order to adjust the clearance between the drum and brake shoes to an appropriate distance after assembling, in other words, it is cumbersome to adjust the clearance between the drum and brake shoes after assembling them to the vehicle.
When the initial clearance between the drum and brake shoes is set small at the time of assembling them to a vehicle in order to solve the problem, the operation of assembling them to the vehicle becomes difficult.
Meanwhile, in a drum brake apparatus having the automatic clearance adjusting device of the conventional one-shot adjuster type, if the initial clearance set between the drum and brake shoes at the time of assembling them to a vehicle is large, braking operation needs only be repeated once or a few times in order to adjust the clearance between the drum and brake shoes to an appropriate distance after assembling. Therefore, the operability in assembling them to the vehicle can be improved and the clearance between the drum and brake shoes can simply be adjusted after assembling.
In the conventional one-shot adjuster type, there is a possibility that the drum can be deformed for example by excessive braking force. If the deformation of the drum temporally increases a movable range of the brake shoes, the position of the brake shoes is adjusted in accordance with the increased movable range. Therefore, the clearance between the drum and the brake shoes to be secured in a non-braked state can be less than an appropriate distance, in other words, over-adjusting can happen. This may cause the brake shoes to be dragged.